Online has always been Forza's strength. The game offers something for everyone from the hard core to the casual to the drifters. You can jump in pre-configured hoppers (vote driven) or public/private races (host driven) along with a new feature called Rivals.
Rivals is not on online race but is a really fantastic addition worth mentioning. It is more of a super advance leaderboard system for a bunch of custom events. It pits you against your nearest friend's ghost (or nearest player if you don't have a friend who has done the event). You put your best time or drift score up against them and the game keeps everyone up to date with an in-game message center.
This year the online field is expanded to 16 cars which is just insane. You can also have up to 11 ai drivers as part of your 16 car field. There is a slew of host options for online races in host controlled rooms. You can setup everything from a multiclass race to a game of cat&mouse. Not only does the host have a ton of race options but they can also restrict just about any other assist or car setting as well.
Online you can choose from cars in your garage, your clubs garage or any stock car from the game. In many races a tuned car from your garage is going to be a slight advantage just because it can be as close to the class cut off as possible. That is not to say you cannot be competitive in a stock car but it is best to take the stock cars that are at the upper end of each class. You can also enter races where stock is a forced option so everyone is on equal terms.
Have a look about half way down this page and you can find some details about Monthly Rivals and the Turn10 Race Hoppers:
http://forzamotorsport.net/en-us/wir_10-7/